By means of steam, hydrocarbons can catalytically be converted to synthesis gas, i.e. mixtures of hydrogen (H2) and carbon monoxide (CO). As is explained in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, keyword “Gas Production”, this so-called steam reformation is the most frequently used method for the production of synthesis gas, which subsequently can be converted to further important basic chemicals such as methanol or ammonia. Although different hydrocarbons, such as naphtha, liquefied gas or refinery gases, can be converted, steam reforming with methane-containing natural gas is dominant.
The steam reformation of natural gas is strongly endothermal. Therefore, it is carried out in a reformer furnace in which numerous catalyst-containing reformer tubes are arranged in parallel, in which the steam reforming reaction takes place. The outer walls of the reformer furnace are lined or provided with several layers of refractory material which withstands temperatures up to 1200° C. The reformer tubes mostly are fired by means of burners which are mounted on the upper or lower surface of the reformer furnace and directly fire the space between the reformer tubes. The heat transfer to the reformer tubes is effected by thermal radiation and convective heat transmission of the hot flue gases.
After preheating by heat exchangers or fired heaters to about 500° C., the hydrocarbon-steam mixture enters into the reformer tubes after final heating to about 500 to 800° C. and is converted there at the reforming catalyst to obtain carbon monoxide and hydrogen. Nickel-based reforming catalysts are widely used. While higher hydrocarbons are completely converted to carbon monoxide and hydrogen, a partial conversion usually is effected in the case of methane. The composition of the product gas is determined by the reaction equilibrium; beside carbon monoxide and hydrogen, the product gas therefore also contains non-converted methane and steam.
After leaving the reformer furnace, the hot synthesis-gas product gas is cooled in one or more heat exchangers by indirect heat exchange against a stream of water. The stream of water is evaporated and can be released as high-pressure steam to loads inside and outside the reforming plant. In the last-mentioned case, the vapor stream released is referred to as export steam. On cooling of the hot flue gases, further high-pressure steam is obtained, which likewise can be released as export steam. The partly cooled synthesis-gas product gas subsequently undergoes further conditioning steps which are dependent on the type of product desired or on the succeeding process.
In an efficient way, steam reforming plants therefore produce two main products, namely synthesis gas and a product vapor stream possibly consisting of several partial vapor streams, which wholly or partly is released as export steam. The loads consuming the synthesis gas product (e.g. hydrogen generation, oxosynthesis, generation of CO) typically are not identical with the loads consuming the export steam. It is disadvantageous that discrepancies therefore can occur between the synthesis gas production and the consumption of the export steam, since the production rates of the two products synthesis gas and product vapor are coupled to each other and the production rate of the export steam only can be adapted to a very small extent. In particular in partial load operation of the steam reformer, the export steam quantity committed to external loads possibly cannot be maintained. Furthermore, it may be desirable to temporarily lower the export steam quantity to be released, for example in the case of the shutdown of a plant or in partial load operation of consuming plants.
Methods for the steam reformation of methane therefore have been described already, in which it has been attempted to achieve an uncoupling of the export steam production from the synthesis gas production. The European Patent Application EP 2103568 A2 for example describes a method for the steam reformation of methane, in which virtually no export steam is released to external loads. This is achieved in that on the one hand virtually the entire steam produced is consumed in the reforming process itself. On the other hand, the steam generation for example is minimized in that the fuel is burnt in the reformer furnace by means of oxygen-enriched air, whereby the mass flow of hot combustion waste gases available for the indirect heat exchange with water/vapor streams is reduced. Furthermore, it is proposed to use the heat quantity transmitted by indirect heat exchange against hot synthesis-gas product gas or hot combustion waste gases for overheating a vapor stream which subsequently is utilized for energy generation in a steam turbine. The energy obtained thereby in turn can be used for oxygen generation or oxygen enrichment of the combustion air. What is disadvantageous here is the expensive plant concept.
A similar approach for minimizing the steam export is adopted in the patent specification EP 2103569 A2. Beside the largely complete consumption of the generated steam within the reformer, the cooled synthesis-gas product gas is supplied to a pressure swing adsorption in which carbon dioxide is separated. The obtained product gas of the pressure swing adsorption, which is enriched in hydrogen and therefore has an increased calorific value, is partly recirculated to the reformer furnace and used there as fuel. This is disadvantageous, because the valuable product hydrogen generated at high cost merely is thermally utilized by combustion.
In Hydrocarbon Engineering (2001), 6(8), pp. 47-50, Grant et al. describe a novel steam reforming plant for hydrogen production as well as the problems occurring in its construction as regards the environmental requirements to be observed and the limitation of the erection site. It is stated that by employing a novel type of reformer in combination with a pre-reformer unit for example a reduction of the export steam could be achieved.
The opposite case, however, in which a stable, high export steam quantity also becomes possible with a variable load condition or partial load operation of the steam reformer, so far has not been considered in the prior art. Especially this mode of operation, however, is relevant in integrated industrial complexes, in which the steam reformer is embedded in numerous adjacent plants consuming export steam. Here, a stable release of export steam is of high importance for the economy of the entire process. Due to the above-described disadvantages there is furthermore a demand for an alternative method, in which even in full load operation of the reformer the export steam quantity released can at least temporarily be decreased.
The recirculation of a part of the flue gas which is obtained by combustion of the burner feedstock in the burners of the reformer furnace to the burners is a measure used in the steam reformation of hydrocarbons. In the German laid-open publication DE 2513499 A, for example, a novel reformer furnace is proposed, in which the heat transmission to the reformer tubes substantially is effected only by convection and only for a minor part by thermal radiation. Beside other constructive measures, this is achieved in that a part of the flue gas is recirculated to the furnace after an optional re-compression, mixed with fuel and combustion air, and recirculated to the burners. The partial recirculation of the flue gases to a combustion furthermore is a known measure in the combustion power plant technology, where it is employed for reducing the emission of CO and nitrogen oxides.